Engine exhaust often contains incomplete combustion compounds such as hydrocarbons, carbon monoxide (CO) and nitrogen oxides (NOx). These compounds have to be removed from engine exhaust for air pollution control and to satisfy various government regulations. There are various systems that have been created for various types of engines and fuel configurations to address the challenging emission control problems. These include three way catalysts, close-coupled catalysts, as well as filters or catalyzed filters. Most of these catalysts or combined catalyst systems are based on precious metals, including Pt, Pd and Rh. Although these noble metal catalysts are effective for mobile emission control and have been commercialized in industry, precious metals are extremely expensive. This high cost remains a critical factor for wide spread applications of these catalysts. There is a constant need of alternative, cheaper catalysts for the effective removal of HC, CO and NOx compounds from mobile emission sources in order to meet increasingly stringent regulations.
One possible alternative has been the use of base metals. Base metals are abundant and much less costly than the precious metals. Several attempts have been made to develop base metal based catalysts for emission control. However, each of these attempts has been fraught with problems. For example, some monolith catalysts have been made that result in the formation of AB2O4 and perovskite type crystal ABO3. However, formation of perovskite structure significantly reduces the catalyst surface area. In other attempts, Cr has been used. However, Cr is highly toxic. Base metal formulations containing both Zn and Cr are likely to lead to catalyst deactivation as a result of Zn loss and regulatory barrier, due to toxicity of Cr. Other base metal catalysts have simply not been able to achieve acceptable levels of pollutant reduction.
In a carbureted motorcycle engine, wide ranges of air to fuel ratios are often encountered as a result of loose control by the carburetor. An emission control catalyst is therefore required to function in this wide range of environments and often loses CO conversion activity under rich aging conditions. Carbureted motorcycle engine emission is characterized with oscillating gas compositions and flow rates (volume) during various driving cycles. Under so-called “rich conditions,” the air-to-fuel ratio of the exhaust is less than the stoichiometric ratio required for complete oxidation of hydrocarbon and CO and reduction of NOx. Similarly, under what is known in the art as “lean conditions,” there is excess air supplied, which provides more than enough oxygen for CO and hydrocarbon oxidation. However under lean conditions, there is insufficient reductant for NOx reduction.
Additionally, the temperature of engine emission may vary depending on the stage of the driving cycle, type of fuel, and engine technologies. Emission gas also contains steam as a combustion byproduct at a level of about 10%. Thus, to simultaneously convert HC, CO and NOx under both rich and lean conditions, water activation is critical. Under rich conditions, steam reforming of hydrocarbons and water gas shift reaction can make up the deficiency in oxidant (O2). Similarly, the reforming and water gas shift reactions can produce more efficient reductant (H2) than hydrocarbons and CO for NOx conversion under lean conditions.
Thus, there is a need for a TWC-containing catalyst article with improved CO conversion performance and stability after hydrothermal aging, particularly under rich engine operating conditions. There is also a need for an affordable, yet effective, catalyst. In particular, there is a need for such a catalyst for carbureted motorcycle engine applications.